No philosophical intuition has a longer history than that which divides sensible qualities into two kinds, primary and secondary. Something like it appears in Democritus, nearly 2500 years ago, and has been continuously maintained in some form or another ever since then. Philosophers today largely continue to think that there is something right about the distinction, even while it remains notoriously difficult to find agreement on just where its ultimate basis lies. As Mark Johnston (1992) puts it, the primary–secondary distinction has “the dubious distinction of being better understood in extension rather than intension. Most of us can generate two lists under the two headings, but the principles by which the lists are generated are controversial, even obscure” (229). I hope to shed some light on this obscure question. My thesis, in brief, is that the secondary qualities are those qualities of objects that bear a certain relation to our sensory powers: roughly, they are those qualities that we can readily detect only through a certain distinctive phenomenal experience. Contrary to what is sometimes supposed, there is nothing about the world itself (independent of our minds) that determines the distinction between primary and secondary qualities. Instead, a theory of the secondary qualities must be grounded in facts about how we conceive of these qualities, and ultimately in facts about human perception